Concrete countertops have many pluses and few minuses. They are durable, heat resistant, and relatively inexpensive. But most of all, they are highly attractive and a great fit with contemporary styles.

Once used exclusively for outdoor building projects, concrete has expanded its range to become a premier material for indoor construction as well. Still utilized mostly to cast countertops and vanity tops, concrete is continually finding new applications inside the home, including fireplace hearths, floors, and even furnishings. Along with remarkable strength and extreme durability, concrete has charm and appeal unlike any other building material.

A concrete countertop may be cast in place or formed offsite and installed like a natural stone countertop. For a number of reasons, casting offsite makes more sense for most of us. In addition to keeping the mess and dust out of your living spaces, working in a garage or even outdoors lets you cast the countertops with the finished surface face-down in the form. This way, if you do a careful job building the form, you can keep the grinding and polishing to a bare minimum. In some cases, you may even be able to simply remove the countertop from the form, flip it over, and install it essentially as is.

Thorough planning and careful form construction are the keys to a successful concrete countertop project. One of the first issues to tackle is weight: concrete weighs about 140 pounds per cubic foot (roughly 25 pounds per square foot for a 2" thick countertop). Most floors should be able to support a heavy countertop, but be sure to inspect floor joists and framing, especially in older homes, to determine if any reinforcement is needed. If you are unsure, consult a building professional, your local building inspector’s office or a structural engineer.

The weight of the concrete is also a factor when it comes to cabinetry. Typical base cabinets should be reinforced at the back and across the top with 1⁄2" or ¾" plywood. Reinforcing cabinetry may increase the overall dimensions, which can cause problems with modular units or in areas with limited space.

After you design your project and determine the actual dimensions, you’ll need to estimate the amount of concrete you’ll need. Concrete is measured by volume in cubic feet; multiply the length by the wide and then by the thickness of the finished countertop for volume in cubic inches, then divide the sum by 1728 for cubic feet. For example, a countertop that will be 48-in.-long × 24-in.-wide × 31⁄2-in.-thick will require 21⁄3-cu.-ft. of mixed concrete (48 × 24 × 3.5 / 1728 = 21⁄3). The best way to achieve consistent results when mixing concrete is to use premixed materials. One 60-lb. bag of premixed high-strength concrete equals 1⁄2-cu.-ft. of mixed concrete. A number of online concrete outlets also offer a virtual rainbow of dry-mix color pigments that are formulated with a water reducer admixture. Water reducers limit the amounts of water the concrete mix uses to help produce a stronger mix and a smoother finished produce.

As you mix the concrete materials, blend all dry ingredients thoroughly in a motorized mixer for five minutes prior to adding liquid ingredients. Do not mix the concrete until the form is completely built, with any sink or faucet knockouts and the reinforcement in place. For best results, mix the concrete in a single batch with a power mixer. Because the mixing container on power mixtures should never be more than half full (one-third full on some models), you’ll need a relatively large mixer for all but the smallest countertops. For the island countertop projects shown here, a tow-behind nine cubic foot mixer was rented (yet another good reason for casting your countertop offsite). Once your casting is done, let the concrete cure for at least a week before you strip off the forms.

Step-by-Step How to Pour a Kitchen Island Concrete Countertop

Cut 3⁄4" melamine-coated particleboard into 21⁄4"-wide strips to make the form sides. Cut the sides to length (331⁄2 and 411⁄2", as shown here) and drill two countersunk pilot holes 3⁄8" in from the ends of the front and back form sides. Assemble the strips into a frame by driving a 2" coarse wallboard screws at each pilot hole and into the mating ends of the end form strips.

Mount a power drill in a right-angle drill guide (or use a drill press) and drill 1⁄4"-dia. pilot holes for 3" deck screws at 6" intervals all the way through the tops of the form sides. Countersink the holes so the screw heads are recessed slightly from the surface.

Center the form frame on the base, which should have the melamine side facing up. Test the corners with a carpenter’s square to make sure they’re square. Drive one 31⁄2" deck screw per form side, near the middle. The screw heads should be slightly below the top edges of the forms. Check for square again, and continue driving the 31⁄2" screws at 6" intervals through the pilot holes. Check for square frequently––the stress easily can pull the frame out of joint.

Make the sink knockout (if you’re installing a sink). The sink we used requires a 143⁄8" square cutout with corners that are rounded at a 1⁄2" radius. Cut three pieces of 3⁄4”-thick MDF to 143⁄8" square using a table saw if possible. With a compass, mark 1½"-radius curves at each corner for trimming. Make the trim cuts with a jigsaw (only if you stack and cut all three at once). Cut just outside the trim line and sand up to it with a sander for a smooth curve.

Use the rounded knockout blank as a template for marking and cutting the other two blanks to match. Clamp the three blanks together and sand the edges smooth and even. A belt sander on a stationary sanding station or an oscillating spindle sander works great for this. If you are strong and steady-handed, you can freehand the corners with a portable belt sander. Don’t over-sand—this will cause the sink knockout to be too small.

Install the sink knockout. Because gluing the faces together can add height to the knockout (and cause the concrete finishing tools to bang into it when they ride on the form tops), attach each blank directly to the layer below it using countersunk screws. Keep the edges aligned perfectly, especially if you’re planning to install an undermount sink.

Concrete Countertop Faucet Knockouts
Option: If your sink faucet will not be mounted on the sink deck, you’ll need to add knockouts for the faucet hole or holes, according to the requirements of the faucet manufacturer. You can order knockouts from a concrete countertop supplies distributor or create them with PVC plumbing pipe that has an outside diameter equal to the required faucet hole size. To anchor the PVC knockout, cover one end with a cap made for that size tubing. Drill a guide hole through the center of the cap so you can secure it with a screw. Position the knockout next to a form side and compare heights. If the knockout is taller, trim the uncapped tubing.

Seal any exposed edges with fast-drying polyurethane varnish, and then caulk the form once the varnish is dry. Run a very thin bead of colored silicone caulk (the coloring allows you to see where the caulk has been laid on the white melamine) in all the seams and then smooth carefully with a fingertip. In addition to keeping the wet concrete from seeping into gaps in the form, the caulk will create a slight roundover on the edges of the concrete. The smoother the caulk, the less grinding you’ll have to do later. If you will be installing an undermount sink, also caulk around the sink knockout.

Add reinforcement to the form. For thicker castings, bend #3 (3⁄8") rebar to fit around the perimeter of the form using a rebar bender. The rebar needs to be at least 1" away from all edges (including knockouts) and 1" away from the top surface. Metal reinforcement can telegraph through concrete if it is too close to the visible surfaces. Tie the ends of the rebar with wire and set it in the form on temporary 1" spacers.

Hang the rebar loop with wires to suspend it, keeping it at least 1⁄2" away from the concrete surface. Drive a few screws into the outside faces of the form sides near the top and use wire ties to hang the rebar. Once all of the ties are in place, remove the temporary spacers. For extra insurance, add another reinforcing material, such as wire poultry netting, over the rebar and tie the two together.

To keep the form from moving during the all-important pouring, finishing, and curing stages, attach or clamp the scrap plywood work top to the actual top. Check for level and insert shims between the work top and the bench top if needed. When possible, drive screws up through the bench top and into the work top to hold it steady. Otherwise, use clamps and check them regularly to make sure they’re tight. If you’re concerned about mess, put a sheet of 3-mil plastic on the floor.

Add all of the dry ingredients into a concrete mixer that’s large enough to do the whole pour. The dry ingredients include high/early concrete mix rated to 5,000 p.s.i. and synthetic fiber reinforcement. If you are using dry pigment, also add this now. Run the mixer for several minutes to thoroughly blend the dry ingredients.

Blend the wet ingredients (water plus any liquid concrete colorant and a water reducer or acrylic fortifier) in a bucket and, with the mixer running, add them slowly to the dry ingredients. Add more water as necessary until the concrete is thoroughly hydrated, but still stiff.

Fill the countertop form, making sure to pack the concrete into the corners and press it through the reinforcement. Overfill the form slightly.

Vibrate the form vigorously as you work to settle the concrete into all the voids. You can rent a concrete vibrator for this purpose, or simply strike the form repeatedly with a rubber mallet. If you have a helper and a sturdy floor and worktable, lift up and down on the ends of the table, bouncing it on the floor to cause vibrations (this is a very effective method if you can manage it safely). Make sure the table remains level when you’re through.

Strike off the excess concrete from the form using a 2 × 4 drawn along the top of the form in a sawing motion. If voids are created, pack them with fresh concrete and restrike. Do not overwork the concrete.

Once you are certain you won’t need to vibrate the form any further, snip the wire ties holding the rebar loop and embed the cut ends attached to the rebar below the concrete surface.

Smooth the surface of the concrete with a metal screeding tool, such as a length of angle iron or square metal tubing. Work slowly with a sawing motion, allowing the bleed water to fill in behind the screed. Since this surface will be the underside of the countertop, no further tooling is required. Cover with sheet plastic and allow the concrete to dry and cure undisturbed for a full week.

Remove the plastic covering and then unscrew and remove the forms. Do not pry against the fresh concrete. In most cases, you’ll need to cut apart the sink knockout to prevent damaging the countertop when removing it. Drill a starter hole and then carefully cut up to the edge of the knockout. Cut the knockout into chunks until you can remove it all (inset). The edges of the concrete will be fragile, so be very careful.

With a helper or two (or three), flip the countertop so the finished surface is exposed. Be extremely careful. The best technique is to roll the countertop onto an edge, position a couple of 2 × 4 sleepers beneath it (insulation board works very well), and then gently lower it onto the sleepers.

Option: To expose the aggregate and create a very polished surface that resembles natural stone, grind the surface. Use a series of increasingly fine diamond-wheel grinding pads (50-grit, then 100, 200, and 400) mounted on a shock-protected 5" angle grinder (variable speed). This is messy work and can go on for hours to get the desired result. Rinse the surface regularly with clean water and make sure it stays wet during grinding. For a gleaming surface, mount still finer pads (up to 1,500 grit) on the grinder and wet-polish.

Clean and seal the concrete with several coats of quality silicone-based concrete sealer (one with penetrating and film-forming agents). For extra protection and a renewable finish, apply a coat of paste wax after the last coat of sealer dries.

Undermount or self-rimming, it is easier to install the sink before the countertop is mounted on the cabinet. Attach the sink according to the manufacturer’s directions. Self-rimming sinks likely will require some modifications to the mounting hardware (or at least you’ll need to buy some extra-long screws) to accommodate the thickness of the countertop.

Make sure the island cabinet is adequately reinforced and that as much plumbing as possible has been installed, then apply a thick bead of panel adhesive or silicone adhesive to the tops of the cabinets and stretchers. With at least one helper, lower the countertop onto the base and position it where you wish. Let the adhesive dry overnight before completing the sink hookup.

Important safety information

To reduce the risk of personal injury and property damage, read and follow all related product manufacturers instruction manuals and information before performing any of the applications shown. Some construction-related applications shown may not be allowed in your local area. Consult your local authority having jurisdiction (AHJ) before starting work for information on permits, applicable codes, and other regulations. Black & Decker disclaims any responsibility for any damage to property or injury to persons as a result of misuse of the information provided.

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